Wire rope, for example made up of steel elements, is used extensively in critical applications, such as in mine hoists or cranes. As a result of use, the strength properties of the wire rope can deteriorate. In some applications, for example, when wire rope is used as a mooring line, the rope can suffer from tension-tension fatigue. That is, the rope is subjected to a cyclic increase and decrease of tension, which detrimentally affects its properties. In other applications, for example, where wire rope is used over pulleys, the wire rope can suffer from bending fatigue in use. That is, the properties of the wire rope deteriorate when the wire rope is subjected repeatedly to bending.
One of the key concerns of a wire rope user is to determine when the wire rope should be replaced. Replacing a wire rope entails substantial costs and effort. These include not only the cost of the new wire rope and labor associated with its replacement, but also the costs associated with down-time of the unit in which the wire rope is used. Therefore, it is undesirable to replace a wire rope too soon, that is, substantially before the end of its useable lifetime. On the other hand, the situation that a wire rope breaks or otherwise fails is unacceptable, and needs to be prevented.
Therefore, within the wire rope field methods have been developed to test the properties of the wire rope while it is in use to allow the wire rope user to determine when to retire a given segment or set of wire ropes. Testing the properties of a wire rope while it is in use may be accomplished by non-destructive testing (NDT) methods.
A first non-destructive testing method that may be used for wire rope evaluation is magnetic field testing, wherein the wire rope to be tested is brought into a magnetic field, and the presence of defects in the wire rope is detected through areas of flux leakage. A further method is eddy current testing wherein an alternating electrical current is passed through a coil producing a magnetic field. When the coil is placed near a conductive material, the changing magnetic field induces current flow in the material. These currents travel in closed loops and are known as eddy currents. Eddy currents produce their own magnetic field that can be measured and used to determine the presence of flaws in the wire rope. In general, NDT methods monitor for changes in the wire ropes shape and geometry over time, as well as for localized breaks within the elements of the wire rope, all of which are indicative of wear and damage to the wire rope. The NDT methods can indicate when predetermined damage thresholds are passed such that the wire rope requires replacement.
In addition to the above, wire ropes are also monitored through visual inspections of the outer strands to identify the number and density of broken wires within the rope.